The development of an organic EL device is being actively pursued from the viewpoints of the applications to displays and lighting. The driving principle of the organic EL device is as described below. That is, holes and electrons are injected from an anode and a cathode, respectively. The holes and electrons are transported through an organic thin film and recombine with each other in a light-emitting layer to form an excited state. Then, the excited state emits light. In order to increase luminous efficiency, the holes and electrons must be efficiently injected and transported through the organic thin film. However, the transfer of carriers in the organic EL device is restricted by an energy barrier between an electrode and an organic thin film and a low carrier mobility in the organic thin film. Thus, there is a limitation on an improvement in luminous efficiency.
Methods which have been devised to solve such problem include a method involving improving hole-injecting property from an anode and transporting holes to a light-emitting layer at a lower voltage by the insertion of a hole-injecting layer between an anode and a hole-transporting layer.
For example, Patent Document 1 discloses that the use of a phthalocyanine-based metal complex in the hole-injecting layer allows for a reduction in driving voltage and an improvement in driving stability of a device. However, there has been a problem in that luminous efficiency is lowered because the phthalocyanine-based metal complex has absorption in a visible light region. Further, there has also been a problem in that the chromaticity is difficult to be controlled in color development.
Further, Patent Document 2 discloses an organic EL device provided with an n-p junction layer formed of an n-type organic layer adjacent to an anode and a p-type organic layer provided on the n-type organic layer. Further, Patent Document 2 discloses an organic electroluminescent device in which the difference between the LUMO energy level of the n-type organic layer and the Fermi energy level of the anode is 2.0 eV or less, and the difference between the LUMO energy level of the n-type organic layer and the HOMO energy level of the p-type organic layer is 1.0 eV or less. Here, the n-type organic layer can be interpreted as a hole-injecting layer. Further, the p-type organic layer can be interpreted as a hole-transporting layer or a light-emitting layer.
In addition, Patent Document 2 discloses, as an electron donating compound used for the n-type organic layer, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), fluorine-substituted 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA), cyano-substituted PTCDA, naphthalenetetracarboxylic acid dianhydride (NTCDA), fluorine-substituted NTCDA, cyano-substituted NTCDA, or hexanitrile hexaazatriphenylene (HAT).    Patent Document 1: JP 63-295695 A    Patent Document 2: WO 2005/109542 A1